1. Field of the Invention
The present invention relates to an air-fuel ratio control system for controlling the air-fuel ratio (A/F) of an air-fuel mixture to be supplied to an internal combustion engine.
2. Related Art
There has been proposed a linear A/F sensor utilizing the oxygen concentration cell capability and oxygen ion pumping capability of zirconia, for detecting whether the air-fuel ratio is on a leaner or richer side of a stoichiometric ratio and also for detecting the value of the air-fuel ratio (see Japanese Laid-Open Patent Publication No. 63(1988)-36140).
One conventional linear A/F sensor will be described below with reference to FIGS. 17 through 20 of the accompanying drawings. FIG. 17 shows a linear A/F sensor including a sensor cell 20 and a pump cell 21 which are shown detached from each other, and each comprise a stabilized zirconia device. The sensor cell 20 and the pump cell 21 are coupled to each other through an insulation layer 22. The sensor cell 20 and the pump cell 21 have respective diffusion holes 23 and 24 defined therein for passing therethrough exhaust gases from an internal combustion engine. The insulation layer 22 has a detecting cavity 25 defined therein into which exhaust gases, can be introduced through the diffusion holes 23 and 24 by the sensor cell 20 and the pump cell 21. The diffusion holes 23 and 24 and the detecting cavity 25 jointly serve as an element for controlling the speed at which the exhaust gases are diffused. The insulation layer 22 also has a reference chamber 25a positioned below the detecting cavity 25 in spaced-apart relation thereto, where the reference chamber 25a is defined between the sensor cell 20 and the pump cell 21. A reference gas such as atmospheric air is introduced into the reference chamber 25a through a communication hole (not shown). As shown in FIG. 18, the sensor cell 20 has porous electrodes 26, 27 of platinum, and the pump cell 21 has porous electrodes 28 and 29 of platinum, where the electrodes 26, 27, 28 and 29 double as catalysts. The sensor cell 20 has an electric heater 30 for heating itself to a temperature range, e.g., 800.degree..+-.100.degree. C. in order to keep the sensor cell 20 active.
The sensor cell 20 functions as a conventional O.sub.2 sensor for developing an electromotive force if there is an oxygen concentration difference between the electrodes 26 and 27. The pump cell 21 also has the same properties as the sensor cell 20, and serves to pump oxygen from a negative electrode to a positive electrode when an electric current (pump current Ip) is caused to flow between the electrodes 28 and 29.
A control assembly 31 detects an electromotive force Vs developed by the sensor cell 20, and also controls the pump current Ip through a feedback loop in order to keep constant the electromotive force Vs, i.e., in order to keep an oxygen concentration corresponding to a stoichiometric ratio in the cavity 25 or the diffusion holes 23 and 24. Since the pump current Ip continuously varies with respect to the air-fuel ratio, as shown in FIG. 19, the air-fuel ratio can be calculated from the pump current Ip.
More specifically, the control assembly 31 includes a comparator 1 and an integrator amplifier 2 with positive and negative power supplies. The comparator 1 compares the electromotive force Vs and a reference voltage Vref corresponding to the stoichiometric ratio. The output signal from the comparator 1 is integrated by the integrator amplifier 2, whose integral output signal is applied as the pump current Ip to the pump cell 21 through a resistor 5. At this time, a voltage drop across the resistor 5 is detected by a current detector 3 which produces a voltage signal commensurate with the pump current Ip. Therefore, the pump current Ip is detected indirectly by the current detector 3. The output signal of the current detector 3 is applied to an adder 4 which then produces an output signal Vout, in the range of 0 to 5 volts, as representing the air-fuel ratio, according to the following equation: EQU Vout=G.multidot.Ip+Vstp,
where G is the current-to-voltage conversion gain of a current-to-voltage converter which is composed of the resistor 5 and the current detector 3, and Vstp is a step-up voltage in the range of 0 to 5 volts.
In the conventional system shown in FIG. 18, the voltage drop across the resistor 5 is applied to a current inversion detector 6 to detect the direction in which the pump current flows, thereby producing a stoichiometric air-fuel ratio Vstc (see FIG. 20).
The air-fuel ratio of an internal combustion engine is controlled by a feedback control loop so as to achieve a target air-fuel ratio based on the air-fuel ration information produced by an air-fuel ratio sensor. For example, when the air-fuel ratio is controlled within a narrow range or within a window close to the stoichiometric air-fuel ratio, the three-way catalytic converter in the exhaust system can operate highly efficiently. With a lean-burn engine having a lean-NOx catalytic converter and a three-way catalytic converter in the exhaust system, the air-fuel ratio is controlled by a feedback control loop so as to achieve a target air-fuel ratio, i.e., a certain leaner value, based on the air-fuel ratio information from a linear A/F sensor.
Accurate control of the air-fuel ratio so that it reaches a target value while the internal combustion engine is in operation is very important for improved fuel economy, increased engine output power, a more stable idling engine speed, purified exhaust emission, and improved drivability. It is necessary that the linear A/F sensor which produces the air-fuel ratio information be controlled so as not to be thermally deteriorated and destructed due to blackening.
Air-fuel ratio sensors, particularly a linear A/F sensor, are complex in structure, and should be composed of a heater, a sensor cell, and a pump cell in combination for operation.
If the linear A/F sensor, or its pump cell, in particular, fails to operate, then the air-fuel ratio signal Vout and the stoichiometric ratio signal Vstc tend to deviate from their true values, and the air-fuel ratio information produced by the linear A/F sensor becomes low in reliability.
Therefore, in the event of a failure of the linear A/F sensor, it is desirable that the failure be detected early, the air-fuel ratio feedback control process based on the sensor output be stopped, and another air-fuel ratio control process be carried out instead.
It is also necessary for accurate air-fuel ratio control that the air-fuel ratio information be stably produced at all times by the linear A/F sensor.
The air-fuel ratio signal Vout produced by the linear A/F sensor poses no problem insofar as the sensor operates in a stoichiometric air-fuel mixture atmosphere. However, if the linear A/F sensor operates continuously under a leaner air-fuel mixture atmosphere, then the air fuel ratio signal Vout thereof is liable to vary with time as shown in FIG. 14.
More specifically, if the engine operates continuously with the air-fuel ratio controlled for a certain leaner target air-fuel ratio, the air-fuel ratio signal Vout produced by the linear A/F sensor tends to become lower with time. It is known that when the engine is raced to shift the air-fuel ratio temporarily toward a richer side, the pump current changes its direction in the period ER (FIG. 14), and the air-fuel ratio then regains the same value as that at the starting time ST, i.e., the O.sub.2 detecting characteristics are regarded as being recovered, at the end of the period ER.
At the time the output signal from the linear A/F sensor indicates some trouble, therefore, the air-fuel ratio information produced thereby becomes less reliable.
In the event of a failure of the linear A/F sensor, therefore, it is desirable to determine whether the sensor is being subjected to a malfunction from which it can be recovered, or a failure from which it cannot be recovered, so that any subsequent air-fuel ratio feedback control process may be interrupted or another air-fuel ratio feedback control process may be selected instead of the feedback control process.